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and V5+, respectively. Fig. 1(a) shows the preparation process and the digital photos of five kinds of vanadium electrolytes (in 1 mm cuvette). The required solutions were then placed in sealed glass jars to be used. 2.2. Static stability measurements The static stability of all five kinds of vanadium electrolytes was tested in a thermostat (Guangdong HongZhan Technology Co. Ltd, PU-80) under various temperatures range from -35 C to 50 C. The temperature deviation of the thermostat was 0.5 C. The electrolytes were put in sealed volumetric flasks and examined every day. When the electrolyte appeared precipitation at low temperature, then, it was warmed to room temperature or higher temperature to see if the precipitation could dissolve again. Similarly, if any electrolyte appeared deposition at high temperature, it was cooled to room temperature or lower temperature to see if the process is reversible. Three groups of samples were tested for the stability study, the results were repeated and the photos were taken by a digital camera outside the thermostat. 2.3. Conductivity and viscosity measurements Conductivity and viscosity of each electrolyte was measured respectively on a conductivity meter (Mettler Toledo AG, SevenEasy S30, Switzerland) and Ubbelohde viscometer at various temperatures. Electrolyte was put in the thermostat for enough time to make sure the temperature reached the set value and then measured by the conductivity meter as quickly as possible. The Ubbelohde viscometer with the electrolyte was put in the thermostat and the connected pipes were extended outside the thermostat for operation. The kinematic viscosity was tested when the thermostat was kept at a set temperature for an hour. Viscosity of the deionized water was used as the reference at each temperature. The viscosity of V(II) electrolyte was measured in N2 gas atmosphere to avoid oxidation. For the conductivity and viscosity tests, three parallel tests were conducted and the average values were presented. 2.4. Electrochemical measurements Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS, frequency range from 105–102 Hz with amplitude of 10 mV) measurements were performed on PARSTAT 2273 electrochemical workstation (Princeton Applied Research). A classic three-electrode system was used in the measurement, which consisted of a glass carbon electrode with 3 mm diameter (used in CV test) or a graphite stick with 6 mm diameter (used in EIS test) as working electrode, a graphite plate as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The glassy carbon electrode was firstly polished on chamois with 0.3 mm alumina, then cleaned in 1 M H2SO4 through CV from 0 to 1.0V (vs. SCE) for 5 cycles, and finally cleaned by ultrasonic in deionized H2O for 10 min. The CV test was carried out from 0 to 1.6 V vs. SCE at different scan rates in 1.5 M V(IV) (positive electrolyte), while from -1.4 to 0.2 V vs. SCE at different scan rates in 1.5 M V(III) (negative electrolyte) at various temperatures. All the CV measurements were taken three times on each temperature and the deviations were less than 0.5%. Therefore, the CV data was reproducible. The EIS test was implemented in the same positive and negative solutions. In addition, all the potentials are referred to SCE unless otherwise stated. Similarly, the electrolyte had been kept in the thermostat for enough time at each test temperature. 3. Results and discussion 3.1. Charge-discharge process analysis The four kinds of pure vanadium electrolyte used in the experiment were achieved by charging the initial V3.5+ electrolyte (0.75 M V(III) + 0.75 M V(IV) + 2 M H2SO4) in a VRFB single-cell [36,37]. The dissociation constants of H2SO4 at 25C are 1.0103 (K1) and 1.02102 (K2). The actual value of the proton S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 527 Fig. 2. (a) Photographs of five types of electrolytes at low temperature; (b) Photographs of re-dissolution of the precipitation that formed at low temperature.PDF Image | Broad temperature adaptability vanadium redox flow battery
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